The Cell PDF

The Cell The Cell Wall Cell wall one of the most important structures of the cell, it's present in prokaryote and eukaryote except animals. Cell walls protect the cell, maintain its shape, and prevent excessive uptake or loss of water. Plants, fungi and most proti...

The Cell The Cell Wall Cell wall one of the most important structures of the cell, it's present in prokaryote and eukaryote except animals. Cell walls protect the cell, maintain its shape, and prevent excessive uptake or loss of water. Plants, fungi and most protists have cell walls but with a chemical structure different from bacterial cell walls. I. Bacterial Cell Wall: Most bacterial cells are encased by a strong layer called cell wall. It's one of the most important parts of a prokaryotic cell for several reasons, that give them shape and protect them from osmotic lysis, wall shape and strength is primarily due to peptidoglycan. The wall can protect a cell from toxic substances and is the site of action of several antibiotics. This cell wall is composed of peptidoglycan which consists of a carbohydrate matrix (polymers of sugars: N_acetylglucosamine and N_acetylmuramic acid) that is cross-linked by short polypeptide units. The Gr+ cell wall consists of a single homogenous peptidoglycan or (murein) layer 20 to 80 nm thick lying outside the plasma membrane. In contrast, the Gr_ cell wall is quite complex. It has a 2 to 7 nm peptidoglycan layer surrounded by a 7 to8 nm thick outer membrane. A space is seen between the plasma membrane and the outer membrane in Gr_ bacteria, this space is called the periplasmic space (filled with a loose network of peptidoglycan). 1 The Cell Although Archaea may be either Gr+ or Gr_, their cell walls are distinctive in structure and chemical composition. The walls lack peptidoglycan and are composed of proteins, glycoproteins, or polysaccharides. II. Fungal Cell Walls: The cell walls of fungi are mainly consist from chitin (polysaccharides), which is modified cellulose consisting of linked glucose units to which nitrogen groups have been added. II. Plant Cell Walls: The cell wall is the strong, surrounding layer of a plasma membrane. The walls range from 0.1 to over 10 nm in thick and are composed mainly of polysaccharides. The most characteristic component of plant cell walls is cellulose. Another important components of the walls of many kinds of cells are lignin, cutin , suberin and waxes (are fatty substances found in the walls of the outer protective tissues). There are two layer of plant cell walls are the middle lamella (intercellular substance) and primary wall. 2 The Cell The middle lamella: Are usually formed soon after mitosis at such a location to divide the mother cell into two daughter cells. The siting of the new wall is directed by a cluster of microtubules. Walls may increase in thickness as new micro fibrils are deposited on the inner face of outer cell wall layer that contains polysaccharides called pectin’s. Primary cell wall: layer formed between the middle lamella and plasma membrane in growing plant cells. It is primarily composed of cellulose microfibrils within a gel-like matrix of hemicellulose fibers and pectin polysaccharides. a primary wall may lose its ability to grow. Primary walls are often about 0.1 nm thick. Secondary cell walls: This wall happen after the primary cell wall stopped in growing and deposit the microfibrils, so secondary wall formation. The secondary wall is deposited of lignin, cutin and suberin on the inner face of the primary wall, and are often much thicker than primary walls often 10–20 nm in thick. Plant cell wall function The functions of the plant cell wall include: 1- Support: the cell wall provides mechanical strength and support. 2- Regulate growth: sends signals for the cell to enter the cell cycle in order to divide and grow. 3- Regulate diffusion: the cell wall is porous allowing some substances, including proteins, to pass into the cell while keeping other substances out. 4- Communication: cells communicate with one another via plasmodesmata. 3 The Cell 5- Protection: protect against plant viruses and other pathogens. It also helps to prevent water loss. 6- Storage: stores carbohydrates for use in plant growth, especially in seeds. Plasmodesmata: Plasmodesmata (singular, plasmodesma) are small channels that directly connect the cytoplasm of neighboring plant cells to each other, establishing living bridges between cells. The plasmodesmata, which penetrate both the primary and secondary cell walls, allow certain molecules to pass directly from one cell to another and are important in cellular communication. IV. Extracellular matrix in animal cells: Animal cells lack the cell walls; instead, animal cells secrete an elaborate mixture of glycoproteins into the space around them, forming the extracellular matrix (ECM). 4 The Cell Cell membrane (Plasma membrane) Every living cell is covered by a thin, elastic and semipermeable membrane called plasma membrane; it regulates the continuous movement substances into and out of the cell. It must permit the entry of food, oxygen and exit of metabolic wastes, it also maintains the concentration of water, inorganic ions and organic molecules between the cell and the environment. Plasma membrane also protects the cell give the shape to the cell. The basic structure of a biological membrane approximately half the mass is phospholipid, which spontaneously organizes to form a lipid bilayer. Phospholipids contain both hydrophobic and hydrophilic end, the bilayer character of membranes represent the most stable arrangement of lipid molecules in an aqueous environment. The basic structure of a phospholipid includes three kinds of subunits: 1- Glycerol. 2- Fatty acids 3- Phosphate group. (PO43-) 5 The Cell Figure : components of phospholipid unit Membrane proteins All the membranes of the cell, including the plasma membrane, also contain proteins. These may be tightly associated with the membrane and extracted from it only with great difficulty: 1- Peripheral proteins, are attached to the surface of the cell membrane on both the internal and external surface, these may be hormone receptors and enzyme. 2- Integral proteins are embedded in the phospholipid bilayer itself, these are often associated with transporting of molecules from one side to another of membrane, and referred to as carrier proteins. Integral membrane proteins are often glycoproteins that is have sugar residues attached on the side facing the extracellular medium. 6 The Cell Figure : Model of plasma membrane Biological Function of cell membrane: 1- Offers protection to the cell 2- It gives a definite shape to the cell 3- It gives rise to certain organelles 4- It gives rise to locomotor structure like cilia and flagella. 5- It acts as a semipermeable membrane allowing specific molecules to move in and out of the cell. 7 The Cell Transport across Cell Membranes The mechanism involved in the transport of substances can be treated under two broad categories. The first type includes physical mechanism in which the forces that drive the substances across the membrane supplied from the environment of the cell, and is often termed passive transport. The other types includes complex reactions which utilize the free energy produced by cells own metabolism. Passive Transport Diffusion: is the movement of molecules from an area of higher concentration to an area of lower concentration. This difference in the concentration of 8 The Cell molecules across a distance is called a concentration gradient. Simple Diffusion Simple diffusion: is the movement of molecules through a cell membrane from an area of higher concentration on one side of the membrane to an area of lower concentration on the other side that depends on the size and type of molecule and on the chemical nature of the membrane. Molecules that can dissolve in lipids may pass directly through the membrane by diffusion. For example, because of their non-polar nature, both carbon dioxide and oxygen dissolve in lipids. Molecules that are very small but not soluble in lipids may diffuse across the membrane by moving through the pores in the membrane. Osmosis. When two aqueous solution of different concentration are separated by a membrane permeable to water but impermeable to solute molecules, water diffuse through the membrane from low concentration of water molecules until the concentration of either side are equal. These process of solvent movement is called osmosis. 9 The Cell Facilitate Diffusion Another type of passive transport is called facilitated diffusion. A variety of molecules such as sugar and amino acids can get across the cell membrane but not against a concentration gradient, without requiring immediate expenditure energy. This is seen in case of blood cells, muscle cells or liver cells. These kinetics are very similar to those that can be applied to enzymic processes, so we must assume a more complex process than that of simple diffusion to be operating in passive transport. Active transport 10 The Cell Diffusion of substances takes place across the membrane from high to low concentration, but the membrane are also able to effect the transport of soluble up the gradient, in the direction of increasing concentration. Normally cells have a high concentration of K+ ions, which is maintained at all times the cell is in a resting potential. To do so, the cell has to continuously translocate a solute or K + ions into the cell by active transport mechanism at the expense of metabolic energy. Features of Active Transport Mechanism Active transport systems have the following characteristics: 1- these system depend on the metabolic processes yielding ATP. 2- these are solute specific. 3- their activity depends upon the concentration of substances being transported. 4- these are direction specific. 5- these can be selectively poisoned. 6- as a result of integrated action of active transport mechanism. 11 The Cell Some model of active transport Active transport of glucose. In many cells like erythrocytes, glucose can be transported by simple facilitated and diffusion, but in intestinal cells and kidney glucose is absorbed against the concentration gradient by an active transport mechanism. The movement of glucose in the intestinal epithelial cells is coupled to the diffusion of sodium ions down concentration gradient. The mechanism is illustrated in which the carrier proteins binds both sodium and glucose. Sodium ions concentration inside the cell remain low, the carrier release sodium ions inside thus triggering a conformational change in the proteins so that it loses affinity of glucose, thus glucose is released into the cell against concentration gradient. Transport of K+ and Na+ (K+ and Na+ pump). In most cells K+ concentration is relatively high and constant, whereas Na+ concentration is quite low. K+ concentration in the cell is high because of its role in carrying out vital enzymic reaction. Such high and low concentration of K + and Na+ are made possible because of active transport system that can pump K+ into the 12 The Cell cell and Na+ out of the cell. It has been suggested that pumping out of Na + is necessary function to pump in K+ ,glucose and amino acids into the cell. So inhibition of Na+ would also inhibit entry of K+, glucose and amino acids into the cells, (in animal cell). When three sodium ions are extrude, two potassium ions are taken up. Bulk Transport Cells have also developed mechanism to transport across the membrane, larger molecules which otherwise cannot diffuse through the membrane barrier. Generally molecules like proteins and hormones are transported out of the cell by a process called exocytosis. Similarly particulate matter from the cell surface can be transported inside the cell by process called endocytosis. Endocytosis is essentially an energy dependent process, whereas in exocytosis there is no energy expenditure. Exocytosis In exocytosis, material synthesized within the cell that has been packaged into membrane-bound vesicles is exported from the cell following the fusion of the 13 The Cell vesicles with the external cell membrane. The materials exported are cell-specific protein products, neurotransmitters, and a variety of other molecules. Endocytosis In endocytosis, the cell membrane engulfs portions of the external medium, forms an almost complete sphere around it, and then draws the membrane-bounded vesicle, called an endosome, into the cell. Several types of endocytosis have been distinguished phagocytosis, pinocytosis and receptor-mediated endocytosis, in which material binds to a specific receptor on the external face of the cell membrane, triggering the process by which it is engulfed. Cholesterol enters cells by the last route. Phagocytosis 14 The Cell Phagocytosis is a process by which certain living cells called phagocytes, that engulfment of solid particles with the aid of plasma membrane has been observed in leukocytes. The phagocyte may be a free-living one-celled organism, such as an amoeba. In some forms of animal life, such as amoebas and sponges, phagocytosis is a means of feeding. In higher animals phagocytosis is chiefly a defensive reaction against infection and invasion of the body by foreign substances (antigens). Pinocytosis Pinocytosis is a process which refers to a kind of cell drinking; the mode of intake of fluid materials was demonstrated in many type of the cell especially in erythroblast, fluid in contact with the plasma membrane move through narrow channels formed temporarily as deep invagination into a cell interior. From the tips of these channels small vesicles containing the fluid droplets are budded off, which empty the content in cytoplasm. 15 The Cell receptor-mediated endocytosis. It is an extremely selective process of importing materials into the cell. This specificity is mediated by receptor proteins located on depressed areas of the cell membrane called coated pits. The cytosolic surface of coated pits is covered by coat proteins. In receptor-mediated endocytosis, the cell will only take in an extracellular molecule if it binds to its specific receptor protein on the cell’s surface. Once bound, the coated pit on which the bound receptor protein is located 16 The Cell ORGANELLES BOUNDED BY DOUBLE-MEMBRANE ENVELOPES Three of the major cell organelles, the nucleus, mitochondria and in plant cells the chloroplast, share two distinctive features. They are all enclosed within an envelope consisting of two parallel membranes and they all contain the genetic material (DNA). The Nucleus The nucleus is the most essential part of cell which directs and controls all its cellular activities. It was discovered by Robert Brown in 1831. During the life span of the cell, the nucleus exists in two phase, the interphase and division phase. The nucleus is found in all eukaryotic cells. However, certain eukaryotic cell such as the mature sieve tube of higher plants and mammalian erythrocytes contain no nucleus. In such cells nuclei are present only during the early stages of development. The prokaryotic cells do not have true nucleus, the nuclear material is called nucleoid. Shape and size Its shape varies. In spheroid, cuboid or polyhedral cells, the nucleus is generally spherical. Irregular nuclei are found in some leukocytes. The size of the interphase nucleus is different in the cells of different organism and even in different in the same cell. The size of nucleus is related to the chromosome number. Nucleus envelope The nuclear membrane or envelope composed of two membranes with nuclear pores are intervals. The outer membrane is continuous with endoplasmic 17 The Cell reticulum. The space between the two membranes is called perinuclear space. The nuclear space is filled with nucleoplasm. The nucleoplasm contains many enzymes in it. The nucleus contains one or more spherical bodies inside called nucleoi. (nucleolus singular). The nucleolus is rich in Ribonucleic acid (RNA). The chromatin is found in the form of a mesh work in the interphase, this represents the chromosomes that are highly extended. During cell division the chromatin condenses into chromosome. Function of nucleus 1- Nucleus controls the activity of the cell. 2- Nucleus is the seat of heredity since the chromosome contain the DNA. 3- DNA synthesis and replication takes place in the nucleus. 4- RNA synthesis takes place in the nucleus. 5- Nuclear membrane permits exchange of materials between the nucleus and cytoplasm. 6- Transcription of genetic code takes place in the nucleus. Mitochondria Mitochondria were first described by Richard Altmann in 1890who called him as bioblasts. The present name mitochondria were coined by Benda in 1897. Location Mitochondria are universal present in all eukaryotic aerobic cells; they are uniformly distribution in cytoplasm. However, in some case they are located near the nucleus, whereas in others they may be accumulated in the peripheral 18 The Cell cytoplasm, in general mitochondria are located near such structure where energy requirement are the heaviest. For example in muscle cells the concentration of mitochondria is heaviest since these have to be engaged for the rapid supply of much ATP. Total absence of mitochondria in prokaryotes is indicative of their anaerobic life. The number of mitochondria per cell in eukaryotes cell varies from cell to cell and is related to the functional stage of the cell. Shape and Size Within the cell mitochondria may assume different shape ranging from granular to filamentous depending upon the functional state of the cell. Their size also varies approximating the size of bacterium. Structure The structure of mitochondria has been given by plada, according to whom they consist of smooth outer membrane, separated by a space from an inner membrane. The inner membrane is thrown into folds or invagination called crista, which extend into the matrix, the mitochondria lumen. The crista are irregular shaped, some are villose while other are finger-like. The matrix is filled with a dense proteinaceous materials packed with dense granules. The inner surface of the crista is packed with lollipop-like particles attached through short stalk. These particles are called elementary particles. Function 19 The Cell Mitochondria are rich in various types of enzymes, hence they are engaged in a variety of function, as many as 70 enzymes and coenzymes are present in them which are engaged in metabolic function such as; 1- Oxidative phosphorylation 2- Electron transport chain 3- Kreps cycle 4- Oxidation of fatty acids 5- Ion transport Mitochondrial DNA The mitochondrial DNA (mt DNA) is circular and occurs in one or more copies attached to the inner membrane within the matrix. The function of mt DNA is similar to that of nuclear DNA of eukaryotes in the production of rRNA, mRNA, tRNA and proteins. The amount of RNA associated with the mitochondria is at least 20 times more than DNA, and almost all species of tRNA molecules are present. The presence of mRNA and ribosomes in the mitochondria suggest a complete protein synthesizing machinery present inside them. Plastids Plastid are cellular organelles found only in the plant cells, they are surrounded by double membrane, which are readily visible under microscope. Plastid vary in size, number, shape and chemical composition with tissues and organism concerd. There are three main types of plastids which are; 20 The Cell Chloroplast, Chromoplasts and leucoplasts. Chloroplasts These are green plastids and of great biological importance. This conversion is carried out by phototrophs the green plants processing photosynthetic pigments. The ability of plant cells to photosynthesis is located in special organelles called chloroplasts. Shape and Size The number of chloroplast per cell is variable so also their size and shape. In higher plants there may be as many as 40 chloroplast per cell. However in certain case they may be ovoid, discoid or spheroid. Generally they occupy regions adjacent to intracellular air space to facilitate gaseous exchange. Sunlight affects their shapes as will size, resulting in the alteration of their volume. Structure of chloroplasts Fundamental, the chloroplast comprise two distinct parts: 1- The envelope: the envelope is composed of double membrane, the outer membrane separated from the inner membrane by an intermediate space. The outer membrane is permeable to a wide variety of compound of low molecular weight. The inner membrane is a functional barrier between the cytosol and the stroma of the chloroplast. It is impermeable to a number of compound such as sugar phosphate and nucleotide but permeable to CO2. 21 The Cell 2- Stroma: It is heterogeneous matrix enclosed by the inner membrane, and contains a number of granules called grana. A series of internal membranes occupy the stroma and are arranged in the form of flattened discs having connections with their lumen and the surrounding stroma, these are called thylakoids which contain chlorophyll. These carry out photochemical reactions of photosynthesis, and the associated electron transport reaction which lead to the liberation of oxygen, ATP formation and reduction of NADP. The stroma also houses a variety of enzymes necessary to carry out metabolic activity of CO2 assimilation. A gel fluid fills up most of stroma space which contains particles rich in RNA, proteins, lipids, carbohydrates and DNA. Some particles freely in stroma are called plastoclobuli, which are site for lipids storage. Function of chloroplasts 1- The main function of chloroplast is photosynthesis. 2- Chloroplasts exhipit a certain degree of functional autonomy within the intracellular environment. Chloroplast DNA The chloroplast DNA (ctDNA) is present in the stroma which is used to synthesis chloroplast-specific proteins utilizing ribosomes, which are also present in stroma. The ctDNA is circular and present in multiple copies. The DNA is capable of the coding about 150 proteins that are necessary for photosynthesis and synthesis of carbohydrate and lipids. 22 The Cell Endoplasmic reticulum It is present majority of eukaryotic cell. Mature erythrocyte of mammalian and the prokaryotic cells do not have endoplasmic reticulum. It is an organelle present in the cell existing in the form of a network and hence the name endoplasmic reticulum. It was first observed by Keith porter. It takes its origin either from the plasma membrane or from nuclear membrane. It can be defined as a system of membrane-bound space filled with cytoplasm. Chemistry it is made up pf proteins, carbohydrates, lipids and enzymes. The endoplasmic reticulum when found as flattened vesicles they are called cisterna, when they are found as swollen spherical structure they are called vesicles and they are known as tubules when they are tubular and branched. Forms of Endoplasmic Reticulum 1- Rough Endoplasmic Reticulum (RER) In this form ribosomes are found attached on the outer surface giving rise to a rough or granular appearance and hence they are also called granular endoplasmic reticulum. They are found in cells where active protein synthesis is takes place. 2- Smooth Endoplasmic Reticulum (SER) The membranous channels in this type are tubular in appearance and they lack ribosomes in their surface due to which appears smooth and hence the name 23 The Cell smooth Endoplasmic Reticulum. It also called Agranular Endoplasmic Reticulum. This type is found in muscle cells. Functions Endoplasmic Reticulum 1- Endoplasmic reticulum helps in metabolism of carbohydrates, fatty acids, phospholipids and glycolipids. 2- It provides surface for various enzymatic reaction. 3- It stores calcium in muscle cells of animals. 4- It helps in wall formation during division. ? 5- Endoplasmic reticulum helps in the synthesis of nuclear envelop during cell division. 6- Rough Endoplasmic reticulum provides the external surface for attachment of ribosomes and increase the protein synthesis. Ribosomes Ribosomes are small, dense, spherical bodies found in large numbers through the cytoplasm of living cells in both prokaryotic and eukaryotic. These are not enclosed in any membrane and are sites of protein synthesis. They are found attached to Endoplasmic reticulum or free in cytoplasm. They are two basic types of ribosomes called 70S found in prokaryotes and 80S occur in the cytoplasm of eukaryotes. Chemically ribosomes consist of protein and RNA. Each ribosome consist of two sub unit one large and the other smaller sub unit. The large sub-unit which has depression on the surface in two sites called “A 24 The Cell site” it receives the amino acids for the synthesis of proteins, and “P site” or peptide site that helps in the establishment of peptide bond between two amino acids. Function of Ribosomes The function of ribosomes is the protein synthesis under the control of genetic code. Golgi complex or Golgi apparatus The Golgi complex is usually located near the cell nucleus and in animal cell it is frequently disposed around the centriole pair. It normally consists of numerous sets of membrane bound, smooth surface cisterna. Each cisterna is flat, disc shaped and curved like a shallow bowl. It has lumen which is bounded by a single membrane. Each set of flattened disc shaped cisterna forms a structure that resembles a stack of plates called Golgi stack or dictyosome. A stack typically consists of 6 cisterna which are closely packed and almost parallel. Each cisterna is separated from each other by a thin layer of cytoplasm. Golgi cisterna have continuities of lumen which each other and with smooth endoplasmic reticulum tubules. Function of Golgi complex 1- Golgi is useful in strong the proteins synthesis by the cell. 2- It is useful in secretion of enzyme and hormones. 3- Golgi is useful in the synthesis of polysaccharide. 25 The Cell 4- Golgi is useful in lysosome formation. 5- It is useful in cell wall formation in plant cell. Lysosomes Lysosomes are found in most eukaryotic cells, but are abundant in animal cells. They are bounded by a single membrane and ovoid sacs which take their origin from endoplasmic reticulum and Golgi bodies. They contain unique collection of hydrolytic enzymes. The Golgi vesicle containing the processed enzymes which bud off are called primary lysosomes. The primary lysosomes fuses with the vesicle to form secondary lysosome in which the material taken in by endocytosis is digested. The secondary lysosome with undigested materials is called residual body. Chemically the lysosomes are made up of lipids, hydrolytic enzymes and trace of carbohydrate. Function of lysosomes: 1- They help in autophagy. 2- lysosomes enzymes may also be released outside the cells in the extracellular space to break down extra cellular materials. 3- They help in transporting and disposing of waste materials through the residual bodies. 4- They also help in metamorphosis. 26 The Cell Cell cycle In prokaryotes, which do not have a well-defined nucleus, daughter cell are formed by fission, in which cell components of the parent cell are equally divided. However the situation in eukaryotic cells is different where cells divide by a complex process of mitosis. Eukaryotic cell division takes place through a series of orderly events known as the cell cycle. The cell cycle is divided into two basic phases:  Interphase  Mitoses Phase The interphase is divided into three stages: 1- G1 phase (gap 1): This is the period of active RNA and protein synthesis when the cell is preparing itself for DNA synthesis and chromosome replication, and is perhaps the longest phase lasting about 10 hours. 2- S phase: This is marked by synthesis of DNA and centrosomes, lasting about 9 hours, hence called synthetic phase. 3- G2 phase (gap 2); During this phase, the cell is preparing for cell division and requires high energy input, lasting 4 hours of duration. During this period necessary proteins like tubulin, cyclins etc. are synthesized which are needed for mitosis. Mitosis Mitosis may be defined as the process of cell division during which the cell on maturity divides and gives two daughter cells which will be quantitatively similar to each other and similar to the cell from which they have formed. 27 The Cell Four stages are recognized in mitosis through the whole process is a continuous one. The stages are as follows: 1- Prophase: many visible changes take place in the nucleus and cytoplasm during this first stage of mitosis: 1- The chromosomes become condensed due to coiling; as a result the chromosomes become shortened and thickened. 2- Each duplicated chromosome now contains two chromatids. 3- The chromosomes become distinct unit as a result of condensation. The centromere of each chromosome is seen as a clear and distinct zone. 4- In animal cells the paired centrioles move towards the poles. 5- Towards the end of prophase the nucleolus disappears. 6- Finally at the end of prophase the nuclear membrane breaks down and disappears. 2- Metaphase: During this stage the chromosomes move towards the equator of the cell, attaching themselves to the spindle fibers with the help of their centromeres. This movement of the chromosomes is called metakinesis. The chromosomes now lie at the equator of the cell. The groups of chromosomes in the equatorial region of the spindle are called equatorial plate. The centromeres of the chromosomes lie on the equator while the arms extend in any direction. Towards the end of metaphase the centromere of each chromosome divides so that each chromatid has its own centromere. 28 The Cell 3- Anaphase: This is the shorter of all stages. At this stage the separation of chromatids which are now known as daughter chromosomes move towards the opposite poles of the cell along the spindle fibers. In this movement the centromeres of the chromosomes move first. At the end of anaphase each set of daughter chromosomes reaches the respective poles. 4- Telophase: During telophase there are two groups of chromosomes, one at each pole. At the poles the chromosomes undergo uncoiling and become filament like. The nucleolus reappears. The nuclear membrane reappears from elements of endoplasmic reticulum, the astral fibers and spindle fibers disappear. Cytokinesis The cytoplasm begins dividing by the process of cytokinesis. In animal cells cytokinesis takes place by means of a constriction which starts from the periphery and extends to the interior of the cell. In plant cells cytokinesis is by formation of a cell plate which starts from the interior and extend to the exterior. After the cell plate is formed the primary and secondary walls are deposited on either side of it. The cell plate later become the middle lamella. Significance of mitosis division 1- Help in growth and development of body organs 2- This process is used in replacing old, decaying and dead cells. 3- The process of mitoses help in quantitative and qualitative distribution chromosomes to the daughter cells. 4- asexual reproduction. 29 The Cell Meiosis Meiosis takes place only in sex cells of sexually reproduction animal and plant, Meiosis takes place during the formation of gametes. Meiosis may be defined as the process of cell division taking place during the formation of gametes which involves two division in which the nucleus and cytoplasm divide twice but the chromosomes duplicate only once. There by producing four cells each with haploid number of chromosomes. The two division of meiosis are known as the first meiotic division or heterotypic division and second meiotic division or homeotypic division. The first meiotic is a reduction division and the second one is equational division and is similar to mitosis. First meiotic division or Heterotypic division Meiosis I is also referred to as reductional division as chromosome number is reduced to half. Meiosis occur in four phases. Prophase-I, Metaphase-I, Anaphase-I and Telophase-I. Prophase-I: During this phase chromosomes move in several ways. There is an increase in the volume of nucleus. This is long duration comprising five stages which are follows: 1-Leptonema or Leptotene: Prophase-I begins at the leptonema stage when each chromosome is first seen to have condensed from its interphase. The chromosome appear as long, slender threads, having bead like appearance due to the presence of chromosome. 30 The Cell 2-Zygonema or Zygotene: Each chromosome duplicates in this substage. Homologous chromosomes appear to attract each other and pairing begins at one or more point. This pairing is highly specific and occurs between all homologous chromosomes. The process of pairing is called synapsis. The resulting pair of homologous chromosomes is usually called a bivalent. 3-Pachynema or Pacchytene: This is stage of progressive shortening and coiling occurring once zygonema pairing has been completed. The bivalent become shorter and thicker. At pachynema two sister chromatids of each homologous become clear, there by two sister chromatids of a homologous chromosomes are associated with the two sister chromates of their homologous partner. This process is called as crossing over or recombination. The region where cross over takes place are referred chiasmata. 4-Diplonema or Diplotene: Homologous chromosomes become more thicker and shorter. The attraction between the homologous chromosomes declines, and the chromosomes repel one another and open out into loops, but they are tied up at points where crossing over took place. The chiasmata are very conspicuous in dilpotine stage. Terminalization of chaisma starts. Diakinesis: Separation of bivalents continues and terminalization of chiasma is completed. Bivalents migrate to periphery, the nucleolus disappears, the nuclear membrane breaks down, cell centers and spindle fibers appear. 31 The Cell Metaphase-I: The chromosomes are at their maximum condensed state and appear relatively smooth in outline. The spindle formation is complete and the bivalent arrange themselves at the equator of the spindle. The chromosomes arrange with their centromeres towards the poles and arms towards the equator. The centromere does not divide at the end of metaphase I. Anaphase-I: The centromeres do not divide, they continue to hold sister chromatids together. The centromeres of a bivalent along with the whole chromosomes move towards the opposite poles of the spindle. Thus one chromosome of each homologous pair reaches each pole. Thus half of the chromosomes of parent cell go to each pole. The reduction in number of chromosomes takes place at anaphase- I of meiosis. And each anaphase group consists of haploid number of chromosomes. Telophase-I: The arrival of chromosomes at opposite poles marks the beginning of the telophase and the end of first meiotic division. The chromosomal group at each pole is recognized. The spindle disappears. The nuclear membrane and nucleoli reappear thus forming two daughter nuclei. Cytokinesis: Cleavage in animal or cell wall formation in plants occurs as in mitosis. In many plants there is no cell wall formation or interphase. The cell pass directly from anaphase I in prophase II of second meiotic division. 32 The Cell Homotypic or Second Meiotic Division The process involved in second meiotic divisions are mechanically similar to those of mitosis. They involve separation of chromatids of both daughter cells produced during first meiotic division. The second meiotic division differs from mitosis mainly in the presence of haploid number of chromosomes in the daughter cells. This division is comprises four stages: Prophase II, Metaphase II, Anaphase II and Telophase II. Prophase II: The sister chromatid of each chromosomes begin to condense and become thicker. The nucleoli and nuclear membrane breaks down. The spindle fiber appear. Metaphase II: Bipolar spindle is formed. The haploid chromosomes along their centromere arrange themselves at the equator of the spindle. Each chromosome has two sister chromatid attached at centromere. Anaphase II: The centromere for each chromosome divide separation two sister chromatids from each other. The spindle fibers pull the centromeres. The separated chromatids are now called chromosomes are pulled towards the poles. Telophase II: 33 The Cell Chromosome Structure and Organization Chromosomes are darkly stained structure located in the nucleus. Chromosomes become distinct structure only during cell division especially in metaphase stage. The number of chromosomes in a given species is generally constant. All members of that species have the same diploid number of chromosomes in their somatic cells and the same haploid number in their gametes. The number of chromosomes varies greatly from one species to another (8 in Drosophila, 26 in frog and 46 in human). Genome is the specific term for the haploid set of chromosomes. In a diploid cell there are two haploid sets. Homologous chromosomes are identical in size and carry similar or identical genes in corresponding position. Structure of Chromosome The structure of chromosome is best studied during metaphase and anaphase stages of cell division due to maximal contraction. A chromosomes consist of two identical and spirally coild threads called sister chromatids joind at lightly coloured construction, called centromere. On the basis of number of centromere, the chromosomes are called monocentric when they have only one centromere and when they have two, they are known as dicentric, the polycentric when they have more than two centromere, when they do not have a centromere they are known acentric or holocentric when whole surface acts as centromere. With reference to the position of centromere there are different type of chromosome. Telocentric when the centromere is terminal, Acrocentric when the centromere subterminal, Sub-metacentric when the centromere is near the center 34 The Cell and Metacentric when the centromere is at center dividing the chromatids into two equal halves. Each chromatid contains a single DNA molecule. The chromatids are attached to each other only by centromere. They become separated at the start of anaphase when the sister chromatids migrate to opposite poles. Chromatid and chromosome are two name for the same structure, a single linear DNA molecule with associated proteins. The centromere lies within a thinner segment of the chromosome, called primary construction. Centromere contain repetitive and specific DNA sequences with special proteins bound to them, forming a disc shaped structure called kinetokore. Chemical Composition of Chromosome Chemically each chromosome consist of linear, unbroken, double stranded DNA molecule surrounded by two kinds of protein, the histone and nonhistone. These play an important role in determining the physical structure of the chromosome. The DNA around the core of histone molecules, and the nonhistone are associated with that complex. The nonhistone have a basic structure role in chromosomes. If the histone are removed from chromosomes, the DNA uncoil and displaced from the complex, but a skeleton of nonhistone proteins in the shape of the chromosome remains. The histone are basic proteins which have a net positive charge at the normal pH of a cell thus facilitating their binding to the DNA. The histone molecule consist of arginine and lysine. Five main types of histone associated with eukaryotic DNA. H1, H2A, H2B, H3 and H4. 35 The Cell The non histone provide a structural role and also in the regulation of gene expression. These are acidic protein having negative charge. So these bint to positively charge histone in chromatid. Nucleosome Electron microscope studies showed that a chromatin fibers is formed of a chain repeated unit called nucleosome. Each is formed of a core complex called nu body wrapped by DNA strand. Core complex is formed of histone molecule. The DNA segment between the adjoining nucleosome is called linker DNA. The nucleosome are found at about 200 bp intervals along the DNA molecules. Heterochromatin Heterochromatin defined as those rejoin of the chromosome that remain condensed during interphase and early prophase and form chromosomes. The heterochromatic regions can be observed in condensed chromosomes as region that stain more strongly or weakly than the euchromatic region called positive or negative heterochromatin of the chromosomes. There are two type of heterochromatin. 1- Constitutive Heterochromatin It is permanently in all types of cells. It is the most type of heterochromatin. This type contains highly repeated DNA sequence, called satellite DNA, which might have structural role in chromosomes. Within a chromosome, it has been found to be located at specific region like centromeric, telomeric or the entire chromosome (eg: sex chromosomes). 36 The Cell 2- Facultative Heterochromatin It is condensed only in certain cell types or at special stage of development. Frequently in this type one chromosome of the pair becomes either totally or partially heterochromatic eg: X chromosomes in the mammalian female, one of which is active and remains euchromatic. The other X chromosomes in the female becomes heterochromatic in the course of development from the zygote to embryo. The function of this chromosomes in female is to balance the number of functional X. Giant chromosomes These are exceptionally large chromosomes and they are also known as Giant chromosomes, and there are two types of Giant chromosomes: 1- Polytene chromosomes(salivary gland chromosomes) This is one of the giant chromosomes found in the cells of salivary glands, gut and trachea. Balbiani (1881) reported them for the first time. Because of their numerous chromonemata the giant chromosomes have been named polytene chromosomes. Polytene is achieved by replication of DNA several times without nuclear division. That is several duplication of the chromosomes have taken place without the cell division. Thus all of the chromosomes remain together in the same cell. However homologous chromosomes have not separated from each other and remain together. There for a cell that had eight chromosomes originally still 37 The Cell contains eight chromosomes which are joined together at their centromeres to form start like body called the chromocenter. 2- Lamp brush chromosomes Lamp brush chromosomes were observed by flemming (1882). The chromosomes looked like the brushes used to clean oil lamps and hence they were named as lamp brush chromosomes. They are present in oocytes of shark, amphibian and birds. The main axis of a, lamp brush chromosomes consist of the paired homologous, each composed of two chromatids. Thus there are four strand present in the main axis and each is one double helix of DNA with is associated protein. Paired loops of different size extend laterally from the main axis with which the loops axes are continuous. 38 The Cell The function of lamp brush chromosomes is to produce RNA and protein related to the formation of yolk in the growing egg. Human chromosomes In 1956 Jjio and Levan demonstrated and identified 46 chromosomes in human or 23 pairs of which 22 pairs are autosomes and one pair sex chromosome. In male the sex chromosomes consist of an “X” and “Y”, in female they consist of two “X”. The cell dividing mitotically show high coiled chromosomes at metaphase stage. At this stage the chromosome are “X” shaped if the centromere is in an median position (metacentric) and V shaped if the centromere is near one end (acocentric). Depending upon size and centromere position, the 46 chromosomes have been divided into seven groups (A to G). 39 The Cell For each chromosome in human karyotype, the chromosome are numbered for easy identification. The largest pair of homologous chromosome is designed 1, the next largest 2 and so on. In humans chromosomes from 1 to 22 are called the autosomes to distinguish them from the pair of sex chromosomes. Generally the sex chromosomes constitute pair 23, it does not fit properly in the size scale. The “X” chromosome is large metacentric chromosome and the “Y” chromosome is the smallest chromosome. DNA Replication We know that DNA is a polynucleotide sequence, arranged in two strands that run antiparallel. Some organism like virus have single strand RNA functioning as the genetic material. DNA is unique molecule which has self-replicating property. The reproductive stability of an organism is based on a set of instruction contained in every cell. Since the chromosome of prokaryotic cell made up of DNA, we shall use the term genophore to differentiate it from eukaryotic chromosome. The eukaryotic chromosome, besides DNA contains histone and RNA, but the information material is DNA only. 40 The Cell Mechanism of Replication The basic information contained in the double helical structure of DNA, relevant to its replication is: 1- Complementarity of the two chains: Adenine pair with thymine, and guanine pair with cytosine. The adenine-thymine are held together by two hydrogen bonds and the guanine-cytosine base pair are reinforced by three hydrogen bonds. 2- Opposite polarity of strands: 41 The Cell The two polynucleotide chains that run in opposite direction are said to be antiparallel to each other. Models of Replication Replication involves exact copying of parental DNA duplex, in which the base pairing is complementary to the parent strands. The newly synthesized strand is formed in 5 to 3 direction, and the nucleotide are always added at the 3 end having a free OH group and the newly synthesized fragment are enzymatically joined. There are three possible ways in which DNA replicate: 1- Conservation 2- Dispersive 3- Semi-Conservation DNA Replication is Semi-conservative Replication is the process by which the DNA of the ancestral cell is duplicated, prior to cell division. Upon cell division, each of the descendants will get one complete copy of the DNA that is identical to its predecessor. The first stage in replication is to separate the two DNA strands of the parental DNA molecule. The second stage is to build two new strands, using each of the two original strands as templates. The most fundamental aspect of replication is the base pairing of A with T and of G with C. Each of the separated parental strands of DNA serves as a 42 The Cell template strand for the synthesis of a new complementary strand. The incoming nucleotides for the new strand recognize their partners by base pairing and so are lined up on the template strand (Fig. 5.01). Since A pairs only with T, and since G pairs only with C, the sequence of each original strand dictates the sequence of the new complementary strand. Synthesis of both new strands of DNA occurs at the replication fork that moves along the parental molecule. Amazingly, in E. coli, DNA is made at nearly 1,000 nucleotides per second. The replication fork consists of the zone of DNA where the strands are separated, plus an assemblage of proteins that are responsible for synthesis, sometimes referred to as the replisome. The result of replication is two double stranded DNA molecules, both with sequences identical to the original one. One of these daughter molecules has the original left strand and the other daughter has the original right strand. The pattern of replication is semi- conservative, since each of the progeny conserves half of the original DNA molecule. Replication is similar, but not exactly the same, in prokaryotes and eukaryotes. DNA replication in bacteria will be covered initially, as this process is better understood and is less complicated than the process in eukaryotes. Enzyme Involved in DNA Replication 1- DNA polymerase-I,II and III 2- DNA Ligase 3- DNA Primase 4- Helicase 43 The Cell Gene Expression For a cell to operate, its genes must be expressed. The word “expressed” means that the gene products, whether proteins or RNA molecules, must be made. The DNA molecule that carries the original copy of the genetic information is used to store genetic information but is not used as a direct source of instructions to run the cell. Instead, working copies of the genes, made of RNA, are used. Transcription The transfer of information from DNA to RNA is known as transcription and RNA molecules are therefore sometimes referred to as transcripts. The process of transcription occurs in a similar fashion to that of DNA replication. The DNA helix unwinds and one strand acts as a template for RNA transcription. RNA polymerase enzymes join ribonucleosides together to form a single stranded RNA molecule. The base sequence along the RNA molecule, which determines how the protein is made, is complementary to the template DNA strand and the same as the other, non-template, DNA strand. The non-template strand is therefore referred to as the sense strand and the template strand as the anti-sense strand. When the DNA sequence of a gene is given it relates to that of the sense strand (from 5 to 3 end) rather than the anti-sense strand. 44 The Cell The process of RNA transcription is under the control of DNA sequences in the immediate vicinity of the gene that bind transcription factors to the DNA. Once transcribed, RNA molecules undergo a number of structural modifications necessary for function, that include adding a specialized nucleoside to the 5 end (capping) and a poly(A) tail to the 3 end (polyadenylation). The removal of unwanted internal segments by splicing produces mature RNA. This process occurs in complexes called spliceosomes that consist of several types of snRNA (small nuclear RNA) and many proteins. Several classes of RNA are produced: mRNA (messenger RNA) directs polypeptide synthesis; tRNA (transfer RNA) and rRNA (ribosomal RNA) are involved in translation of mRNA and snRNA is involved in splicing. Translation 45 The Cell After processing, mature mRNA migrates to the cytoplasm where it is translated into a polypeptide product. At either end of the mRNA molecule are untranslated regions that bind and stabilise the RNA but are not translated into the polypeptide. The translation process occurs in association with ribosomes that are composed of rRNA and protein complexes. The assembly of polypeptide chains occurs by the decoding of the mRNA triplets via tRNAs that bind specific amino acids and have an anticodon sequence that enables them to recognise an mRNA codon. Peptide bonds form between the amino acids as the tRNAs are sequentially aligned along the mRNA and translation continues until a stop codon is reached. The primary polypeptide chains produced by the translation process undergo a variety of modifications that include chemical modification, such as phosphorylation or hydroxylation. 46 The Cell Central Dogma The Watson and Crick hypothesis explained that DNA functions as the template for RNA synthesis, hence transcription occur in the nucleus of eukaryotes. Subsequently, RNA molecules move to the cytoplasm and along with the ribosomes determine the arrangement of amino acids in proteins. So this flow of information as the central dogma of molecular biology from DNA to RNA to protein. 47  Genetic Terminology Like other sciences, the science of genetics has its specific terminology and We are giving here certain most common terms which are used more frequently in genetics. Gene. The fundamental physical and functional unit of heredity, which carries information from one generation to the next; a segment of DNA, composed of a transcribed region and a regulatory sequence, that makes possible transcription. Allele (Allelomorph). Alleles are genes controlling the same characteristic (e.g. hair colour) but producing different effects (e.g. black or red), and occupying corresponding positions on homologous chromosomes. Autosome. The chromosomes which are not associated with sex are known as autosomes. Except the sex chromosomes (X) and (Y) other chromosomes are the autosomes. Back cross. The cross of a progeny individual with its parents is known as back cross. Carrier. A heterozygous individual. An individual who possesses a mutant allele but does not express it in the phenotype because of a dominant allelic partner; thus, an individual of genotype Aa is a carrier of a if there is complete dominance of A on a. Codominance. When both the alleles (dominant and recessive) are equally expressed in the hybrid, the phenomenon is known as codominance, e.g. Dominant allele. An allele that expresses its phenotypic effect even when heterozygous with a recessive allele; thus if A is dominant over a; then AA and Aa have the same phenotype. Genotype.The genetic makeup or constitution of an individual, with reference to the traits under consideration, usually expressed by a symbol, e.g., +, DD (tall), dd (short), etc Lethal gene. A gene whose phenotypic effect is sufficiently drastic to kill the bearer. Linkage group. All of the genes located physically on a given chromosome. Mendelian ratio. A ratio of progeny phenotypes reflecting the operation of Mendel’s laws. Mendel’s first law. The two members of a gene pair segregate from each other during meiosis; each gamete has an equal probability of obtaining either member of the gene. Mendel’s second law. The law of independent assortment; unlinked or distantly linked segregating genes pairs behave independently. Phenotype. The appearance or discernible character of an individual, which is dependent on its genetic makeup usually expressed in words. Genotype. All of the genes possessed by an individual constitute its genotype. There are two form of genotype: homozygous. The union of gametes carrying identical alleles produces a homozygous genotype. A homozygote produces only one kind of gamete. heterozygous. The union of gametes carrying different alleles produces a heterozygous genotype. Different kinds of gametes are produced by a heterozygote. Wild type. The genotype or phenotype that is found in nature or in the standard laboratory stock for a given organism. X linkage. The presence of a gene on the X chromosome but not on the Y. X- and -Y linkage. The presence of a gene on both the X and Y chromosomes. Y linkage. The presence of a gene on the Y chromosome but not on the X.  Memndel's Studies Gregor Mende published the results of his genetic studies on the garden pea in 1866 and thereby laid the foundation of modern genetics. In this paper Mendel proposed some basic genetic principles. One of these is known as the principle of Segregation. He found that from any one parent, only one allelic form of a gene is transmitted through a gamete to the offspring. For example, a plant which had a factor (or gene) for round-shaped seed and also an allele for wrinkled- shaped seed would transmit only one of these two alleles through a gamete to its offspring. Mendel knew nothing of chromosomes or meiosis, as they had not yet been discovered. We now know that the physical basis for this principle is in first meiotic anaphase where homologous chromosomes segregate or separate from each other. If the gene for round seed is on one chromosome and its allelic form for wrinkled seed is on the homologous chromosome, then it becomes clear that alleles normally will not be found in the same gamete. Mendel's principle of independent assortment states that the segregation of one factor pair occurs independently of any other factor pair. We know that this is true only for loci on non-homologous chromosomes. For example, on one homologous pair of chromosomes are the seed shape alleles and on another pair of homologues are the alleles for green and yellow seed color. The segregation of the seed shape alleles occurs independently of the segregation of the seed color alleles because each pair of homologues behaves as an independent unit during meiosis. Mendel's experiments : 1- Mendel used the garden pea (Pisum sativum ) as his experimental organism. 2- He examined the inheritance of clear traits like purple flower versus white ,and he study seven antagonistic pairs of traits. 3- He isolated lines of peas that pure. 4- Worked with large number of plants and used statistical analysis.  Mendel's laws : 1- Law of segregation 2- Law of Independent Assortment Law of segregation : Mendel carried out many of monohybrid crosses : mating between individuals that differ in one trait such as seed color , in each monohybrid cross one parent carries one form of trait and the other parent carries the alternative form of the same trait. Mendel planted pure breeding green peas and pure breeding yellow peas and allow them to grow into the parental (P) generation. Later when the plants flowered , he dusted the female stigma of green – pea plant flowers with pollen from yellow pea plant , he also performed reciprocal cross ( reversing the traits of male and female parents) by dusting yellow pea plants stigma with green pea pollen. He found that in both cases the peas were yellow. These yellow peas , progeny of the P generation called first filial (F1) generation.To learn whether the green trait had disappeared entirely or remained intact but hidden in these (F1) yellow peas , he planted them and allowed the (F1) to self – fertilization , he then counted the peas of the second filial (F2) generation , progeny of the (F1) generation. among the progeny there were 6022 yellow and 2001 green , the ratio 3 yellow to 1 green. Parental (pure) ♂ yellow peas × ♀ green peas ↓ First filial (F1) all yellow ↓ Self – fertilization ↓ Second filial (F2) yellow : green He called the trait that appeared in all (F1) hybrids , yellow seeds (dominant) and the green pea trait that hidden in the (F1) but reappeared in the (F2) is ( recessive ) Law of segregation : the two alleles for each trait separate or segregate during gamete formation , then unite at random , one from each parent at fertilization. We write dominant alleles with capital Y and recessive alleles with small y the pure parent YY yellow or yy green. YY can produce only Y gametes, yy produce y gametes , the cross between YY and yy produce Yy hybrid. If you know the genotype and the dominance relation of the alleles you can predict the phenotype. Phenotype : observable characteristic , color of seed. Genotype :alleles of an organism , YY. Homozygous : YY or yy the two copies of gene are the same. Heterozygous (hybrid) : Yy two copies are different. Parental (P1) ♂ yellow peas × ♀ green peas YY ↓ yy Gametes(G1) Y ↓ y y First filial (F1) Yy 100% yellow (hybrid) P2 F1 Yy yellow × F1 Yy yellow ↓ G2 Y y Y y ↓  ♂ Y y ♀ Punnett square Y YY Yy yellow yellow Yy yy y yellow Green F2 yellow green Phenotype 3 : 1 Genotype 1:2:1 Law of independent assortment Mendel explained the inheritance of two traits and how two pairs of alleles would segregate by dihybrid cross , he mated pure plants yellow round seeds YYRR with pure plants green wrinkled seeds yygg. F1 generation YyRr showing only the dominant phenotype , yellow and round F1 dihybrid self – fertilizer produce F2 generation.He found 315 yellow round , 101 yellow wrinkled ,108 round green and 32 wrinkled green.there were new combination of phenotype such yellow wrinkled or green round these called recombination types. Mendel found the second law of genetics. Law of independent assortment : during gamete formation different pairs of alleles segregate independently of each other. The gene for seed and for seed shape assort independently , Y or y with R or r in any gamete. Each dihybrid of F1 generation can make four kinds of gametes : YR ,Yr , yR and yr ,they appear in ratio 1 : 1 : 1 : 1.  P1 ♂ yellow round YYRR × ♀ wrinkled green yygg ↓ G1 YR yr F1 YyRr yellow round 100% P2 F1 YyRr yellow round × F1 YyRr yellow round ↓ G2 YR Yr yR yr YR Yr yR yr ↓ Yr yr ♀ ♂ YR yR YR YYRR YYRr YyRR YyRr yellow round yellow round yellow round yellow round Yr YYRr YYrr YyRr Yyrr yellow round yellow wrinkled yellow round yellow wrinkled yR YyRR YyRr yyRR yyRr yellow round yellow round round green round green YyRr yr yellow round Yyrr yyRr yyrr yellow wrinkled round green wrinkled green F2: yellow round yellow wrinkled round green wrinkled green 9 : 3 : 3 : ALLELIC RELATIONSHIPS 1. Dominant and Recessive Alleles. Whenever one of a pair of alleles can come to phenotypic expression only in a homozygous genotype, we call that allele a recessive factor. The allele that can phenotypically express itself in the heterozygote as well as in the homozygote is called a dominant factor. Upper- and lowercase letters are commonly used to designate dominant and recessive alleles, respectively. Usually the genetic symbol corresponds to the first letter in the name of the abnormal (or mutant) trait. Example. Lack of pigment deposition in the human body is an abnormal recessive trait called "albinism." Using A and a to represent the dominant (normal) allele and the recessive (albino) allele. respectively, 3 genotypes and 2 phenotypes are possible: Genotypes Phenotypes AA (homozygous dominant) Normal (pigment) Aa (heterozygotc) Normal (pigment) Aa (homozygous recessive) Albino (no pigment) Carriers. Recessive alleles are often deleterious to those who possess them in duplicate A heterozygote may appear just as normal as the homozygous dominant genotype. A heterozygous individual who possesses a deleterious recessive allele hidden from phenotypic expression by the dominant normal allele is called a carrier. Most of the deleterious alleles harbored by a population are found in carrier individuals. 2. Codominant Alleles. Alleles that lack dominant and recessive relationships may be called incompletely dominant, partially dominant, semidominam or codominanl. This means that each allele is capable of some degree of expression when in the heterozygous condition. Hence the heterozygous genotype gives rise to a phenotype distinctly different from either of the homozygous genotypes. Usually the heterozygous phenotype resulting from codominance is intermediate in character between those produced by the homozygous genotypes. Example. The alleles governing the M-N blood group system in humans are codominanis and may be represented by the symbols LM and LN. Two anti-sera (anti-M and anti-N) arc used to distinguish three genotypes and their corresponding phenotypes (blood groups). Agglutination is represented by + and nonagglutinution by -. Reaction with Blood groups Genotype Anti - M Anti - N Phenotype LM LM + - M LM LN + + MN LN LN - + N 3- MULTIPLE ALLELES If the mutant allele has developed from the wild form of allele due to mutation, one may expect that the wild form of allele can mutate in more than one way. The mutant form of allele too can mutate once again to give rise to another mutant form of allele. Therefore, it is possible to have more than two allelic forms, i.e., multiple alleles, of one kind of gene. Although only two actual alleles of a gene can exist in a diploid cell (and only one in a haploid cell), the total number of possible different allelic forms that might exist in a population of individuals is often quite large. This situation is termed as multiple allelism, and the set of alleles itself is called a multiple allelic series. The most important and distinguishing features of multiple alleles are summarized below : 1.Multiple alleles of a series always occupy the same locus in the chromosome. 2.Because, all the alleles of multiple series occupy same locus in chromosome, therefore, no crossing-over occurs within the alleles of a same multiple allele series. 3.Multiple alleles always influence the same character. 4.The wild type allele is nearly always dominant, while the other  mutant alleles in the series may show dominance or there may be an intermediate phenotypic effect. 5.When any two of the mutant multiple alleles are crossed, the phenotype is mutant type and not the wild type. Example, the human blood groups designated A, B, O, or AB are determined by three types of alleles denoted IA, IB, and IO, and the blood group of any person is determined by the particular pair of alleles present in his or her genotype. 4- LETHAL GENES Lethal genes are mutant genes and result in the death of the individual which carries them. Death of the individual occurs either in the prenatal or postnatal period prior to sexual maturity. A fully (completely) dominant lethal allele kills both in homozygous and heterozygous states. Individuals with a dominant lethal allele die before they can leave progeny. Therefore, the mutant dominant lethal is removed from the population in the same generation in which it arose. Recessive lethal genes kill only when they are in a homozygous state and they may be of two kinds : 1. one which has no obvious phenotypic effect in heterozygotes and 2. one which exhibits a distinctive phenotype when heterozygous. The completely lethal genes usually cause death of the zygote, later in the embryonic development or even after birth or hatching. Complete lethality, thus, is the case where no individuals of a certain genotype attain the age of reproduction. However in many cases lethal genes become operative at the time the individuals become sexually mature. Such lethal genes which handicap but do not destroy their possessor are called subvital, sublethal or semilethal genes. The lethal alleles modify the 3:1 phenotypic ratio into 2 : 1. Examples of Lethal Alleles A. Lethal alleles in plants. In plants, recessive lethal alleles are known which produce albinism, where absence of chlorophyll is lethal (fatal) to them. Following two examples illustrate this fact : In snapdragons (Antirrhinum majus) three types of plants occur : 1. green plants with chlorophyll ; 2. yellowish green plants with carotenoids, usually are referred as pale green, golden or auria plants and 3. white plants without any chlorophyll. The homozygous green plants have the genotype CC and the homozygous white plant has the genotype cc. The auria plants have the genotype Cc because they are heterozygotes of green and white plants. B. Lethal alleles in human beings. In humans several hereditary diseases have lethal effects. Few important lethal genes of man are following : Congenital ichthyosis. One of the most typical cases of a recessive lethal gene in man is expressed in congenital ichtyosis. At birth children afflicted with this disease have a crusted leathery skin with deep fissures down to the subcutaneous tissue; the fissures lead to bleeding, infection and death. Congenital ichthyosis occurs only when there occurs homozygous condition for its recessive lethal genes. SEX-LINKED INHERITANCE Any gene located on the X chromosome (mammals, Drosophila, and others) or on the analogous Z chromosome (in birds and other species with the ZO or ZW mechanism of sex determination) is said to be sex-linked. The first sex-linked gene found in Drosophila was the recessive white-eye mutation. This peculiar type of inheritance is due to the fact that the Y chromosome carries no alleles homologous to those at locus on the X chromosome. In fact, in most organisms with the Y-type chromosome, the Y is virtually devoid of known genes. Thus males carry only one allele for sex-linked traits. This one-allelic condition is termed homozygous in contrast to the homozygous or heterozygous possibilities in the female. VARIATIONS OF SEX LINKAGE The sex chromosomes (X and Y) often are of unequal size, shape, and/or staining qualities. The fact that they pair during meiosis is indication that they contain at least some homologous segments. Genes on the homologous segments are said to be incompletely sex-linked or partially sex-linked and may recombine by crossing over in both sexes just as do the gene loci on homologous autosomes. Genes on the nonhomologous segment of the X chromosome are said to be completely sex linked and exhibit the peculiar mode of inheritance described in the preceding sections. In humans, a few genes are known to reside in the nonhomologous portion of the Y chromosome. In such cases, the trait would be expressed only in males and would always be transmitted from father to son. Such completely Y-linked genes are called holandric genes (Fig.). SEX - INFLUENCED TRAITS The genes governing sex-influenced traits may reside on any of the autosomes or on the homologous portions of the sex chromosomes. The expression of dominance or recessive ness by the alleles of sex influenced loci is reversed in males and females due, in large part, to the difference in the internal environment provided by the sex hormones. Thus examples of sex-influenced traits are most readily found in the higher animals with well- developed endocrine systems. Example: The gene for pattern baldness in humans exhibits dominance in men, but acts recessively in women. SEX-LIMITED TRAITS Some autosomal genes may only come to expression in one of the sexes either because of differences in the internal hormonal environment or because of anatomical dissimilarities. For example, we know that Mammalian have many genes for milk production that they may transmit to their daughters, but they sons are unable to express this trail. The production of milk is therefore limited to variable expression in only the female sex. When the penetrance of a gene in one sex is zero, the trait will be sex- limited.  Chromosomal Mutation-I (Cytogenetics : Changes in Structure of Chromosomes) Genetics makes extensive use of deviations from the norms, and the study of chromosomes is no exception. The chromosomes of each species has a characteristic morphology (structure) and number. But, sometimes due to certain accidents or irregularities at the time of cell division, crossing over or fertilization some alterations in the morphology and number of chromosomes take place. The changes in the genome involving chromosome parts, whole chromosomes, or whole chromosome sets are called chromosome mutations. Chromosome mutations are inherited once they occur and are of the following types : A. Structural changes in chromosomes : 1. Changes in number of genes (a) Loss : deletion (b) Addition : Duplication 2. Changes in gene arrangement : (a) Rotation of a group of genes 180 within one chromosome : inversion (b) Exchange of parts between chromosomes of different pairs : translocation. B. Changes in number of chromosomes : 1. Loss, or gain, of a part of the chromosome set (aneuploidy) 2. Loss, or gain, of whole chromosome set (euploidy) (a) Loss of an entire set of chromosomes (haploidy) (b) Addition of one or more sets of chromosomes (polyploidy). Both types of changes (structural and numerical) in chromosomes can be detected not only with a microscope (cytologically) but also by standard genetic analysis.  STRUCTURAL CHANGES IN CHROMOSOMES For better understanding of the abnormalities of chromosome structure, let us consider two important features of chromosome behaviour : 1- During prophase I of meiosis, homologous regions of chromosomes show a great affinity for pairing and they often go through considerable contortions in order to pair. 2- structural changes usually involve chromosome breakage; the broken chromosome ends are highly “reactive” or “sticky”, showing strong tendency to join with broken ends. Types of Structural Changes in Chromosome Structural changes in chromosome may be of the following types: 1. deficiency or deletion which involves loss of a broken part of a chromosome. 2. duplication involves addition of a part of chromosome (i.e., broken segment becomes attached to a homolog which, thus, bears one block of genes in duplicate). 3. inversion in which broken segment reattached to original chromosome in reverse order, and 4. translocation in which the broken segment becomes attached to a nonhomologous chromosome resulting in new linkage relations. Variation in Chromosome Morphology Various changes in chromosome structure often produce variation in chromosome morphology such as isochromosomes, ring chromosomes and Robertsonian translocation. 1. Isochromosomes. An isochromosome is a chromosome in which both arms are identical. It is thought to arise when a centromere divides in the wrong plane, yielding two daughter chromosomes, each of which carries the information of one arm only but present twice. For example, telocentric X chromosome ofDrosophila may be changed into an “attached-X” which is formed due to misdivision of the centromere. 2. Ring chromosomes. Chromosomes are not always rod-shaped. Occasionally ring chromosomes are encountered in higher organisms. Sometimes breaks occur at each end of the chromosome and broken ends are joined to form a ring chromosome. Crossing over between ring chromosomes can lead to bizarre anaphase. 3. Robertsonian translocation. Thus, Robertsonian translocation is an eucentric reciprocal translocation where the break in one chromosome is near the front of the centromere and the break in the other chromosomes is immediately behind its centromere. The resultant smaller chromosome consists of largely inert heterochromatic material near the centromere; it normally contains no essential genes and tends to become lost. Thus, Robertsonian translocation results in a reduction of the chromosome number.  Chromosomal Mutation-II (Cytogenetics : Changes in Chromosomes Number) Each species has a characteristic number of chromosomes in the nuclei of its gametes and somatic cells. The gametic chromosome number constitutes a basic set of chromosomes called genome. A gamete cell contains single genome and is called haploid. When haploid gametes of both sexes (male and female) unite in the process of fertilization adiploid zygote with two genomes is formed. However, sometimes irregularities occur in nuclear division and causes Changes in number of whole chromosomes is called heteroploidy Heteroploidy may involve entire sets of chromosomes (euploidy), or loss or addition of single whole chromosomes (aneuploidy). ss or addition of single whole chromosomes (aneuploidy). Each may produce phenotypic changes. A. EUPLOIDY Changes in complete sets of chromosomes : 1- Monoploid. One set of chromosomes (1N) is characteristically found in the nuclei of some lower organisms such as fungi. Monoploids in higher organisms are usually smaller and less vigorous than the normal diploids. Few monoploid animals survive. A notable exception exists in male bees and wasps. Monoploid plants are known but are usually sterile. monoploidy is common in plants and rare in animals. 2- Triploid : Three sets of chromosomes (3N) can originate by the union of a monoploid gamete (1N) with a diploid gamete (2N). The extra set of chromosomes of the triploid is distributed in various combinations to the germ cells, resulting in genetically unbalanced gametes. Because of the sterility that characterizes triploids, they are not commonly found in natural populations. 3- Tetraploid : Four sets of chromosomes (4N) can arise in body cells by the somatic doubling of the chromosome number. Doubling is accomplished either spontaneously or it can be induced in high frequency by exposure to chemicals such as the alkaloid colchicine. Tetraploids are also produced by the union of unreduced diploid (2rt) gametes. Colchicine: is a drug and its aqueous solution is found to prevent the formation and organization of spindle fibres, so the metaphase chromosomes of the affected cells (called C-metaphase or colchicine metaphase) do not move to a metaphase plate and remain scattered in the cytoplasm. Even the process of cytokinesis is prevented by colchicine and with duplications of chromosomes the number goes on increasing. As colchicine interferes with spindle formation, its effects are limited to dividing and meristematic cells. Phenotypic Effects of Polyploidy The increase in the genome’s size beyond the diploid level is often caused following detectable phenotypic characteristics in a polyploid organism : (i) Morphological effect of polyploidy. The polyploidy is invariably related with gigantism. (ii) Physiological effect of polyploidy. The ascorbic acid content has been reported to be higher in tetraploid cabbages and tomatoes than in corresponding diploids. (iii) Effect on fertility of polyploidy. The most important effect of polyploidy is that it reduces the fertility of polyploid plants in variable degrees. (iv) Evolution through polyploidy. Interspecific hybridization combined with polyploidy offers a mechanism whereby new species may arise suddenly in natural populations. Polyploidy in humans have been found in liver cells and cancer cells. In them polyploidy is whether complete or as a mosaic, it leads to gross abnormalities and death. 2. Aneuploidy: Variations in chromosome number may occur that do not involve whole sets of chromosomes, but only parts of a set. 1- Monosomic. Diploid organisms that are missing one chromosome of a single pair are monosomics with the genomic formula 2n - I. The single chromosome without a pairing partner may go to either pole during meiosis, but more frequently will lag at anaphase and fails to be included in either nucleus. In animals, loss of one whole chromosome often results in genetic unbalance. which is manifested by high mortality or reduced fertility.